Zinc Iodine Flow Battery

ABSTRACT

A zinc iodine flow battery includes a positive end plate, a positive current collector, a negative current collector, a positive electrode with a flow frame, a membrane, a negative electrode with a flow frame, a negative end plate. The negative electrolyte is circulated between the negative storage tank and the negative cavity by pump. The negative pipe is provided with a branch pipe for the positive electrolyte circulation. The porous membrane between the positive and negative electrodes can realize the conduction of supporting electrolyte and prevent the diffusion of I3− to the negative electrolyte. In a duel-flow battery system, same electrolyte serves as both the positive electrolyte and the negative electrolyte, which is a mixed aqueous solution containing iodized and zinc salt. The membrane is porous membrane does not contain ion exchange group. Both the positive and negative electrolyte are neutral solutions.

TECHNICAL FIELD

The invention relates to the field of flow battery, in particular to thefield of zinc iodine flow battery.

BACKGROUND TECHNOLOGY

The massive consumption of fossil energy has caused energy crisis andenvironmental problems. The development and utilization of renewableenergy has received great attention all over the world. However, thediscontinuity and instability of renewable energy, such as wind andsolar energy, enable the difficult utilization of it, therefore, therealization of the continuous supply of renewable energy via the largescale energy storage technology is the key to solve the above problem.Due to the advantages of flexible design (energy and power are designedseparately), high safety, long cycling life and not limited by terrain,flow battery has become one of the best technologies for large-scaleenergy storage. Among them, all vanadium flow battery has entered thecommercial demonstration stage with its unique technical advantages.

At present, the relatively mature flow system technologies include allvanadium flow batteries, zinc bromine flow batteries, sodium polysulfidebromine and other systems. However, as for vanadium flow battery, thehigh cost, acidity and corrosiveness of electrolyte and the stronglyoxidizing sulfuric acid and VO₂ ⁺ enable high requirement for themembrane; zinc bromine and sodium polysulfide bromine flow battery wouldgenerate corrosive bromine during the charging process. At the sametime, the high vapor pressure, severe volatilization and environmentalpollution of Br₂ need to be further considered.

Zinc iodine flow batteries employ neutral zinc and iodide salt aselectrolyte, which has the advantage of high solubility and energydensity. Compared with Cl₂ and Br₂, iodine is less corrosive. In themeantime, iodine exists in the form of I₃ ⁻ and the vapor pressure ismuch lower, which makes the zinc-iodine flow battery a promising system.Same as common flow batteries, the zinc iodine flow battery (replaced as“zinc iodine dual-flow battery” in PCT) adopts a dual-pump and pipelinedesign. During the charge and discharge process, the positive andnegative electrolytes circulate between the battery cavity andelectrolyte storage tank. However, because batteries require electrolytecirculation systems such as pumps and storage tanks, the energyefficiency of the system is reduced. On the other hand, the batteryauxiliary equipment such as pumps and storage tanks complicate thebattery system and reduce the energy density of the system. Therefore,research on single flow battery based on dual-flow system and reducingthe energy loss of the system is an important way to improve the energyutilization efficiency and energy density of the whole system. Inaddition, currently reported zinc iodine dual-flow batteries usually useexpensive Nafion membranes, but the above-mentioned ion-exchangemembranes could be easily contaminated in the zinc-iodine system,leading to an increase in ohmic resistance and poor cycle stability ofthe battery. In addition, zinc iodine flow batteries use ZnI₂ as theelectrolyte, which is easily oxidized by air to generate ZnOprecipitation. At the same time, I₂ would be desposited on the positiveelectrode which restricted the stability of the electrolyte and then thecycling life. Therefore, the reported working current density is lessthan 10 mA/cm², which lead to a low power density.

SUMMARY OF THE INVENTION

To solve the above problems, the content of the invention is as follows:

A zinc iodine flow battery comprises either a single battery or a stack.The single flow battery includes a porous electrode and cavity on thepositive side which is filled with electrolyte. In a zinc iodinedual-flow battery, the positive or negative electrolyte circulatesinside the battery and in the storage tank through a pump and apipeline. for single flow battery, there is no pump or pipeline on thepositive side, and the electrolyte is stored in the porous electrode andcavity. As for the negative side, the electrolyte in the battery and inthe negative storage tank could be circuited through the pump andpipeline, and pipeline is provided with a branch for the circulation ofthe positive electrolyte. The dual-flow battery also includes positiveand negative electrolyte storage tanks, which contain the positive andnegative electrolyte, respectively.

When the battery is being charged, I⁻ is oxidized to I₃ ⁻ or I₂ on thepositive electrode, and Zn²⁺ on the negative electrode is reduced to Zn;during discharging, the positive electrolyte is reduced to I⁻, and theZn is oxidized to Zn²⁺. The membrane between the positive and negativeelectrodes prevents I₃ ⁻ from migrating to the negative electrode whileconducting the supporting electrolyte.

Compared with the dual-flow battery, the zinc iodine single flow batteryeliminates the positive storage tank and pump on the positive side, thepositive electrolyte is sealed in the positive porous electrode.Furthermore, the negative pipe is provided with a branch pipe forpositive electrolyte circulation. The structure of single flow batteryincludes positive and negative end plates, membrane, positive electrode,negative electrode, current collector, flow frame, pump and pipeline.The structure of the dual-flow battery includes positive end plate,negative end plate, membrane, positive electrode, negative electrode,current collector, flow frame, pump and pipeline.

The positive electrolyte composition includes iodine salt, zinc salt,and the supporting electrolyte. Iodine salt is one or more of CaI₂,MgI₂, KI and NaI, with a concentration of 2-8 mol/L. The activesubstance in the negative electrolyte is one or more of Zn(NO₃)₂, ZnBr₂,ZnSO₄, ZnCl₂, with the concentration of 1-4 mol/L, the molar ratio ofiodine and zinc in the electrolyte of dual-flow battery is 2:1, thesupporting electrolyte of single flow battery is one or more of KCl,KBr, NaCl and the concentration is 1-2 mol/L. Among them, KI is apreferred iodine salt, ZnBr₂ is a preferred zinc salt, KCl is apreferred supporting electrolyte and the concentration is 1 mol/L forthe dual-flow battery.

The electrode material is one of carbon felt, graphite plate, metalplate or carbon cloth. The electrode material is preferably carbon felt.

As for the zinc iodine flow battery, the membrane used for the zinciodine single flow battery is a porous membrane without ion exchangegroups or a composite membrane. The membrane used for a duel-flowbattery is a porous membrane without ion exchange groups or a compositemembrane. The substrate is a porous membrane, which includes one or moreof polyethersulfone (PES), polyethylene (PE), polypropylene (PP),polysulfone (PS), polyetherimide (PEI), and polyvinylidene fluoride(PVDF). The membrane thickness is 100-1000 μm, preferably 500-1000 μm.The pore diameter is about 10-100 nm with the porosity of 30%-70%.Polyethylene (PE) and polypropylene (PP) are preferred porous substrate.In addition, as for zinc iodine single flow battery, the porous membraneis coated with a dense polymer layer to improve the coulomb efficiencyof the battery; the material of which include: polybenzimidazole (PBI),Nafion resin and (polytetrafluoroethylene) PTFE. Nafion resin ispreferred and the thickness of the coating is 1-10 μm.

The Invention has the Following Beneficial Effects:

1. Compared with the dual-flow battery, the structure of zinc iodinesingle flow battery is greatly simplified, which improves the energydensity of the battery. At the same time, the energy loss of the systemis reduced, which improves the energy efficiency of the system. Inaddition, the concentration of electrolyte is very high, which issuitable for single flow battery design; same as dual-flow battery, zinciodide single flow battery solves the strong acid and alkali issue ofelectrolyte and the cost of electrolyte is relatively low; at the sametime, high current density and the power density of battery could alsobe achieved.2. The positive and negative electrolytes are the same, whicheffectively alleviates the crossover issue due to the similar osmoticpressure of the positive and negative electrolytes. Therefore, thecoulomb efficiency could be greatly improved, which effectively reducesthe system maintenance costs caused by electrolyte migration.Furthermore, the electrolyte could be recovered online, which greatlysaves the replacement cost of the electrolyte and demonstrates a goodapplication prospect.3. Iodine and zinc salt could be employed as the reactant of dual-flowbattery with low cost and environment friendliness; the high solubilityof zinc and iodine salt achieved high energy density. Furthermore, thehigh electrochemical activity of electrolyte enables a high currentdensity and power density of the battery; at the same time, thenegligible corrosiveness of electrolyte could greatly reduce theenvironmental burden. The invented zinc iodine flow battery solves theissue of strong acid and alkali of electrolyte, besides, the supportingelectrolyte could improve the conductivity of the electrolyte and thenthe voltage efficiency.4. The low-cost porous membrane replaces the traditional Nafion 115membrane, which greatly reduces the cost of the stack. In addition, theporous structure of the membrane could improve the conduction of neutralions and the current density of the battery can reach 140 mA/cm², whichmeans great improvement in the voltage efficiency. Most importantly, theporous structure of the porous membrane is filled with oxidized I₃ ⁻,which can alleviate the short-circuit issue that caused by zinc dendriteafter overcharging, so the battery could be self-recovered and greatlyimprove the stability of the battery. In addition, Nafion coating caneffectively alleviate I₂/I₃ ⁻ crossover and significantly improve thecoulomb efficiency of single flow battery (higher than 98%).5. The traditional zinc iodine flow battery employs ZnI₂ as thereactant, which tends to be oxidized into ZnO at room temperature andreduces the cycle stability of the battery; replacing ZnI₂ with KIgreatly improves the stability of the positive electrolyte and the priceof KI is much lower than that of ZnI₂, so the cost of the electrolytecould be greatly reduced.6. The introduction of Br by ZnBr₂ could complex with I₂ to form I₂Br⁻that inhibit the precipitation of I₂ when the battery operates at highSOC and high current density, which greatly improves the cyclingstability of the battery.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the structural diagram of the zinc iodine single flow batteryof the invention. Among them, 1 refers to positive and negative bipolarplates; 2 refers to positive and negative current collectors; 3 refersto positive and negative flow frames; 4 is the membrane; 5 refers topositive electrolyte inlet and outlet valves; 6 is the electrolytestorage tank; 7 is pump.

FIG. 2 shows the single battery cycle performance of the zinc iodinesingle flow battery according to example 1; the positive and negativeelectrolytes are ZnBr₂: 4 M, KI: 8 M, KCl: 1M, and the porous membranewith the thickness of 900 μm.

FIG. 3 shows the energy density of the zinc iodine single flow batteryaccording to example 1; the positive and negative electrolyte is ZnBr₂:4 M, KI: 8 M, KCl: 1 M, and the porous membrane with the thickness of900 μm.

FIG. 4 shows the cycle performance of the zinc iodine single flowbattery according to example 3; the positive and negative electrolyte isZnBr₂: 4 M, KI: 8 M, KCl: 1 M, and the porous membrane thickness: 500μm.

FIG. 5 shows the cycle performance of the zinc iodine single flowbattery according to example 5; the positive and negative electrolyte isZnCl₂: 4 M, KI: 8 M, KCl: 1 M, and the porous membrane thickness: 900μm.

FIG. 6 shows the cycle performance of the zinc iodine single flowbattery according to Example 7; the positive and negative electrolyte isZnBr₂: 4 M, Nat 8 M, KCl: 1 M, and the porous membrane thickness: 900μm.

FIG. 7 shows the energy density diagram of the zinc iodine single flowbattery according to Example 7; the positive and negative electrolyte isZnBr₂: 4 M, Nat 8 M, KCl: 1 M, and the porous membrane thickness: 900μm.

FIG. 8 shows the cycle performance of the zinc iodine single flowbattery according to comparative example 2; the positive and negativeelectrolyte is ZnI₂: 4 M, porous membrane thickness: 900 μm.

FIG. 9 shows the cycle performance of the zinc iodine single flowbattery according to comparative example 3; the positive and negativeelectrolytes are ZnBr₂: 4 M, Nat 8 M, KCl: 1 M, Nafion 115 filmthickness: 125 μm.

FIG. 10 shows the cycle performance of the zinc iodine single flowbattery according to comparative example 5; the positive and negativeelectrolyte is ZnBr₂: 4 M, Nat 8 M, KCl: 1 M, and the porous filmthickness is 65 μm.

FIG. 11 is the structural diagram of zinc iodine dual-flow battery usingporous membrane: 1 refers to the positive and negative pumps; 2 refersto the positive and negative electrolyte storage tank; 3 refers to thepositive and negative end plates; 4 refers to the positive and negativecurrent collectors; 5 refers to the positive and negative flow frames; 6is the membrane.

FIG. 12 shows the single battery cycle performance diagram of the zinciodine dual-flow battery according to example 1; the positive andnegative electrolyte is ZnBr₂: 2.5 M, KI: 5 M, KCl: 1 M, and the porousmembrane thickness is 900 μm.

FIG. 13 shows the single battery cycle performance diagram of the zinciodine dual-flow battery according to example 2; the positive andnegative electrolyte is ZnBr₂: 3 M, KI: 6 M, KCl: 1M, the porousmembrane thickness is 900 μm.

FIG. 14 shows the energy density diagram of the single cell of the zinciodine dual-flow battery according to example 1; the positive andnegative electrolyte is ZnBr₂: 2.5 M, KI: 5 M, KCl: 1 M: 1m, the porousmembrane thickness is 900 μm.

FIG. 15 shows the energy density diagram of the single cell of the zinciodine dual-flow battery according to example 2; the positive andnegative electrolyte is ZnBr₂: 3 M, KI: 6 M, KCl: 1M, the porousmembrane thickness is 900 μm.

FIG. 16 shows the single battery cycle performance diagram of the zinciodine dual-flow battery according to example 3; the positive andnegative electrolyte is ZnBr₂: 2m, KI: 4 M, KCl: 1 M, the porousmembrane thickness is 900 μm.

FIG. 17 shows the single cell cycle performance diagram of the zinciodine dual-flow battery according to example 4; the positive andnegative electrolyte is ZnBr₂: 1 M, KI: 2 M, KCl: 1 M, the porousmembrane thickness is 900 μm.

FIG. 18 shows the single battery cycle performance diagram of the zinciodine dual-flow battery according to example 6; the positive andnegative electrolyte is ZnBr₂: 3 M, KI: 6 M, KCl: 1 M, and the thicknessof the porous membrane is 500 μm.

FIG. 19 shows the single battery cycle performance of the zinc iodinedual-flow battery according to example 12; the positive and negativeelectrolyte is ZnSO₄: 3 M, KI: 6 M, KCl: 1 M, and the thickness of theporous membrane is 900 μm.

FIG. 20 single battery cycle performance diagram of zinc iodinedual-flow battery according to example 14; positive and negativeelectrolyte is ZnBr₂: 3 M, KI: 6 M, and the thickness of porous membraneis 900 μm.

FIG. 21 demonstrates the ratio performance diagram of zinc iodinedual-flow battery according to example 4; the structure of the singlecell battery includes successively: positive end plate, positive currentcollector, positive flow frame, membrane, negative flow frame andnegative end plate. The composition of electrolyte in the battery is 2 MKI, 1 M ZnBr₂, and 2 M KCl, flow rate is 10 ml/min, charging currentdensity is 60-140 mA/cm², the battery is terminated by the capacity andvoltage double cut-off: the charging cut-off time is 45 minutes and thevoltage is 1.5 V, discharging cut-off voltage is 0.1 V.

FIG. 22 is a temperature dependent performance diagram of the zinciodine dual-flow battery assembled in example 4. Battery temperaturedependent performance test: the structure of the single battery is asfollows: positive end plate, positive current collector, positive flowframe, membrane, negative flow frame and negative end plate. Thecomposition of electrolyte in the battery is 2 M KI, 1 M ZnBr₂, and 2 MKCl, flow rate is 10 ml/min, charging current density is 80 mA/cm², thebattery is terminated by the capacity and voltage double cut-off: thecharging cut-off time is 45 minutes and the voltage is 1.5 V,discharging cut-off voltage is 0.1 V, temperature range is 10° C.-65° C.

FIG. 23 demonstrates the voltage curve of a single zinc-iodine dual-flowbattery according to example 2. The structure of the single battery isas follows: positive end plate, positive current collector, positiveflow frame, membrane, negative flow frame and negative end plate. Thecomposition of electrolyte in the battery is 6 M KI, 3 M ZnBr₂, and 1 MKCl flow rate is 10 ml/min, charging current is 80 mA/cm², the batteryis terminated by the capacity and voltage double cut-off: the chargingcut-off time is 45 minutes and the voltage is 1.5 V, discharging cut-offvoltage is 0.1 V. Charge for 1 hour until the battery is shortcircuited, then reduce the charging time to 45 mins to continue thebattery cycling.

FIG. 24 demonstrates a voltage curve diagram of a zinc-iodine dual-flowbattery stack according to example 2. The structure of the stack is: apositive electrode end plate, a current collector, nine batteries eachcomprises a positive electrode with flow frame, a membrane, a negativeelectrode with a flow frame, and finally a current collector and anegative electrode end plate connected in series. The electrolytecomposition of the battery is 6 M KI, 3 M ZnBr₂, and 1 M KCl with a flowrate of 10 mL/min. The charging current density was 80 mA/cm² and thecharge cut-off voltage is 13 V with a discharge cut-off voltage of 1 V.Charge for 1 h until the battery is short-circuited, then reduce thecharging time to 45 mins to continuously evaluated the battery.

FIG. 25 is the cyclic performance diagram of the zinc iodine dual-flowbattery stack according to example 2; the stack assembled with ninesingle battery connected in series.

FIG. 26 shows the cycle performance of a single cell zinc iodinedual-flow battery according to comparative example 1; the positive andnegative electrolyte are ZnBr₂: 2.5 M, KI: 5 M, KCl: 1 M Nafion 115membrane with the thickness of 125 μm.

FIG. 27 shows the cycle performance of a single zinc iodine dual-flowbattery according to comparative example 4; the positive and negativeelectrolyte is ZnI₂: 3 M, and the thickness of the porous membrane is900 μm.

FIG. 28 shows the cycle performance of a single zinc iodine dual-flowbattery according to comparative example 5; the positive and negativeelectrolyte is ZnI₂: 3 M, KI: 5 M, KCl: 1M, porous membrane with thethickness of 65 μm

FIG. 29 is the cycle performance diagram of a single cell zinc iodinesingle flow battery according to preferred example 1; the positive andnegative electrolyte is ZnBr₂: 4 M, KI: 8M, KCl: 1 M, the compositemembrane is PE porous membrane substrate with 7 μm Nafion resin coating.

FIG. 30 is the energy density diagram of zinc iodine single flow batteryaccording to preferred example 1; the positive and negative electrolyteis ZnBr₂: 4 M, KI: 8 M, KCl: 1 M, the composite membrane is PE poroussubstrate with 7 μm Nafion resin coating.

FIG. 31 shows the cycle performance diagram of the zinc iodine singleflow battery according to preferred example 2; the positive and negativeelectrolyte is ZnBr₂: 4 M, KI: 8 M, KCl: 1 M, the composite membrane isPE porous substrate with 7 μm Nafion resin coating.

EMBODIMENTS

The evaluation of zinc iodine dual-flow battery and single flow battery:the structure of the single battery include, sequentially, positiveelectrode plate, current collector, carbon felt positive electrode withflow frame, membrane, carbon felt negative electrode with a flow frame,and negative end plate. The flow rate of the electrolyte in the batterywas 10 mL/min, the charging current density was 80 mA/cm², the batterywas terminated by the capacity and voltage double cut-off: the chargingtime was 45 minutes and the voltage was 1.5 V, discharging cut-offvoltage was 0.1 V.

Electrolyte Composition Examples (mol/L) membrane Thickness (μm) CE VEEE 1 8M KI, 4M ZnBr₂, 1M PE 900 96% 80% 77% KCl 2 6M KI, 3M ZnBr₂, 1M PE900 96% 81% 78% KCl 3 8M KI, 4M ZnBr₂, 1M PE 500 91% 80% 73% KCl 4 6MKI, 3M ZnBr₂, 1M PE 500 91% 81% 74% KCl 5 8M KI, 4M ZnCl₂, 1M PE 900 92%71% 65% KCl 6 6M KI, 3M ZnCl₂, 1M PE 900 93% 70% 65% KCl 7 8M NaI, 4MZnBr₂, 1M PE 900 88% 78% 68% KCl 8 6M NaI, 3M ZnBr₂, 1M PE 900 90% 78%70% KCl 9 8M KI, 4M ZnBr₂ PE 900 96% 78% 75% 10 6M KI, 3M ZnBr₂ PE 90097% 78% 76% Preferred 8M KI, 4M ZnBr₂, 1M Composite 900 97% (85%) (82%)example 1 KCl membrane Preferred 8M KI, 4M ZnBr₂, 1M Composite 500 96%(86%) (81%) example 2 KCl membrane

Comparative Electrolyte Composition example (mol/L) membrane Thickness(μm) CE VE EE 1 ZnI₂ 3M PE 900 90% 81% 73% 2 ZnI₂ 4M PE 900 89% 78% 69%3 8M KI, 4M ZnBr₂, 1M KCl Nafion 115 125 99% 70% 69% 4 6M KI, 3M ZnBr₂,1M KCl Nafion 212 50 99% 68% 67% 5 KI 5M, ZnBr₂ 2.5M, 1M KCl PE 65 74%88% 65%

FIGS. 2 to 3 are graphs of cycle performance and energy density of thebattery under the most preferred conditions. With KI/ZnBr₂ as theelectrolyte, the battery assembled with porous membrane achievedexcellent cycle stability. Meanwhile, the application of porous membranegreatly improved the ion conductivity, the working current density ofthe battery can reach 80 mA/cm² with the high power density. At the sametime, the concentration of KI in the electrolyte can reach about 8 M andthe energy density is greater than 90 Wh/L.

Compared with the most preferred example, the battery in FIG. 4 employsa much thinner porous membrane (500 μm), and the coulombic efficiency ofthe battery decreases due to the increase of electrolyte crossover. Theelectrolyte in FIG. 5 employed ZnCl₂ rather than ZnBr₂, the performanceis greatly reduced and the stability is deteriorated. This is due to theinstability of the electrolyte; during charging, the I₂ despositionformed in the positive electrode and ZnCl₂ in the negative electrolytewould hydrolyze and precipitate. In FIG. 6, when NaI was substitutedwith KI, the battery efficiency decreased. In particular, the voltageefficiency drop is mainly caused by the decrease of the electrolyteconductivity, which further decreased the energy density of the batteryin FIG. 7.

FIGS. 8-10 are comparative experiments. FIG. 8 employed ZnI₂ as theelectrolyte of the battery. The decrease of efficiency was mainly due tothe low ion conductivity of the ZnI₂ solution. Further, the batteryperformance is unstable due to the precipitation of electrolyte. FIG. 9employed Nafion 115 membrane for the battery assembly. During the chargeand discharge process, serious membrane fouling occurred on the membranesurface, which intensified the battery polarization and decreased thebattery performance. FIG. 10 used a much thinner porous membrane, thecross-contamination of electrolyte was greatly intensified, and theefficiency of the battery, especially the coulomb efficiency, wasseverely reduced.

A preferred example employed a Nafion-coated composite membrane as themembrane. FIG. 29 shows the performance of a battery that used compositemembrane with the thickness of 900 μm. The electrolyte was a mixedsolution of KI and ZnBr₂. Due to the Donnan exclusion of Nafion coating,the columbic efficiency of the battery was greatly improved. Inaddition, the battery used a thinner composite membrane (500 μm), andthe coulombic efficiency of the battery slightly decreased.

The evaluation of zinc-iodine dual-flow battery and single flow battery:the structure of a single battery contains, sequentially: a positiveelectrode plate, a current collector, a carbon felt positive electrodewith a flow frame, a membrane, and a battery with a flow frame, a carbonfelt negative electrode with a flow frame, and a negative end plate. Theflow rate of the electrolyte in the battery was 10 mL/min, the batterywas terminated by the capacity and voltage double cut-off: the chargingcut-off time was 45 minutes and the voltage was 1.5 V, dischargingcut-off voltage was 0.1 V

Examples Electrolyte (mol/L) membrane Thickness (μm) CE VE EE 1 KI 5M,ZnBr₂ 2.5M, 1M KCl PE 900 94% 85% 80% 2 KI 6M, ZnBr₂ 3M, 1M KCl PE 90094% 85% 80% 3 KI 4M, ZnBr₂ 2M, 2M KCl PE 900 94% 85% 80% 4 KI 2M, ZnBr₂1M, 2M KCl PE 900 94% 86% 81% 5 KI 5M, ZnBr₂ 2.5M, 1M KCl PE 500 87% 86%75% 6 KI 6M, ZnBr₂ 3M, 1M KCl PE 500 86% 86% 74% 7 NaI 5M, ZnBr₂ 2.5M,1M KCl PE 900 94% 83% 78% 8 NaI 6M, ZnBr₂ 3M, 1M KCl PE 900 94% 82% 77%9 KI 5M, ZnCl₂ 2.5M, 1M KCl PE 900 91% 82% 75% 10 KI 6M, ZnCl₂ 3M, 1MKCl PE 900 90% 81% 72% 11 KI 5M, ZnSO₄ 2.5M, 1M KCl PE 900 76% 81% 61%12 KI 6M, ZnSO₄ 3M, 1M KCl PE 900 75% 80% 60% 13 KI 5M, ZnBr₂ 2.5M PE900 95% 83% 79% 14 KI 6M, ZnBr₂ 3M PE 900 95% 83% 79%

Comparative example Electrolyte (mol/L) membrane Thickness (μm) CE VE EE1 KI 5M, ZnBr₂ 2.5M, 1M Nafion 115 125 99% 81% 80% KCl 2 KI 5M, ZnBr₂2.5M, 1M Nafion 212 50 98% 83% 81% KCl 3 ZnI₂ 2.5M PE 900 99% 71% 70% 4ZnI₂ 3M PE 900 98% 70% 68% 5 KI 5M, ZnBr₂ 2.5M, 1M KCl PE 65 74% 88% 65%

FIG. 11-17 show zinc iodine dual-flow batteries that employed ZnBr₂ andKI as the active substance, KCl as the supporting electrolyte with a 900μm porous membrane. The battery can continuously run stably for morethan 1000 cycles at 80 mA/cm². Above all, the energy efficiency isgreater than 80% with the energy density above 80 Wh/L. The advantagesof the above system include: the introduction of Bf in ZnBr₂ can form acomplex agent of I₂Br⁻, thereby inhibiting the precipitation of I₂; thereplacement of traditional ZnI₂ with KI can avoid the formation of zincoxide and hydroxide during the charge and discharge process. Theemployment of porous membranes benefit the conduction of neutral ions,which improves the operating current density and power density of thebattery. In addition, the absence of ion exchange groups in the membranecan greatly reduce the membrane fouling issue and improve the cyclestability of the battery.

Compared with the most preferred example: FIG. 18 employs a thinnerporous membrane which resulted in a drop of performance especially thecoulomb efficiency. This is mainly due to the employment of a thinnermembrane that lead to much more serious cross-contamination. In FIG. 19,ZnSO₄ replaced ZnBr₂ and the voltage efficiency of the battery wasgreatly reduced, which indicates that the sulfate ion affected theelectrochemical kinetic of electrolyte; in FIG. 20, when the supportingelectrolyte was removed, the voltage efficiency of the battery wasreduced slightly. FIGS. 21 to 25 demonstrate that, under preferredconditions, the battery displayed excellent rate performance andtemperature dependent performance; in addition, the porous membranecould eliminate the zinc dendrites formed on the negative electrode asthe pore structure was filled with oxidized I₃ ⁻, which could react withthe zinc dendrite. Therefore, the single battery and the battery stackcan self-recovered after a micro short circuit occur, which greatlyimproves the stability of the battery. Most importantly, the batterystack can continuously run stably for more than 300 cycles at 80 mA/cm².

Compared with the preferred example: Nafion 115 membrane was used forthe battery in FIG. 26. Due to the poor conductivity of the membrane,the voltage efficiency of the battery was lower than that of optimalexample, however, the employment of Nafion 115 membrane greatly reducedthe crossover issue and greatly improved the coulombic efficiency of thebattery. However, the performance of the battery deteriorated sharplyafter 15 cycles, which was due to the serious membrane fouling of theNafion 115 membrane caused by I₂ and Zn dendrite, the membraneresistance increased greatly and the polarization was intensified. FIG.27 used ZnI₂ as the electrolyte and the battery performance was severelydegraded, which was caused by the instability of the positive andnegative electrolytes. The positive electrolyte would form I₂precipitation during the charging process and the negative electrolytewould form zinc oxide and hydroxide. FIG. 28 used a much thinner porousmembrane, the cross-contamination of the electrolyte was intensified andthe coulombic efficiency of the battery was greatly reduced.

1. The zinc iodine flow battery, which related a zinc iodine single flowbattery or a zinc iodine dual-flow battery; The single flow batterycomprises a negative electrolyte storage tank, a zinc iodine single flowbattery includes a single battery or a cell stack that assembled withmore than two single battery circuits in series; The single batterycomprises a positive end plate, a positive current collector, a positiveelectrode with a flow frame, a membrane, a negative electrode with aflow frame, a negative current collector and a negative end plate; theelectrolyte in the negative electrolyte storage tank could be circuitedbetween the negative cavity (the cavity between the membrane and thenegative current collector is called the negative cavity; the negativecavity is provided with a negative inlet and a negative outlet) and theelectrolyte storage tank by pump; the negative electrolyte storage tankis connected with the negative inlet and outlet through the negativeinlet and outlet of pipeline respectively; at the same time, thenegative inlet and outlet of pipe are respectively provided with abranch for the circulation of the positive electrolyte; (the cavitybetween the membrane and the positive current collector is called apositive cavity; the positive cavity is provided with a positive inletand a positive outlet); the branch on the negative inlet of pipeline isconnected with the positive inlet on the positive cavity, and the branchon the negative outlet of pipeline is connected with the positive outleton the positive cavity; The dual-flow battery comprises a single batteryor a stack that assembled with more than two single battery circuits inseries; the single battery comprises a positive end plate, a currentcollector, a positive electrode with a flow frame, a membrane, anegative electrode with a flow frame, a current collector, and anegative end plate; the positive electrolyte in the positive electrolytestorage tank flows through the positive electrode by the pipeline andpump; the negative electrolyte in the negative electrolyte tank couldflows through the negative electrode by the pipeline and pump; thepositive electrolyte and the negative electrolyte are the same, whichare the mixture aqueous solutions of iodine and zinc salt; the membraneis a porous membrane without ion exchange groups or composite membrane.2. The zinc-iodine flow battery according to claim 1, wherein themembranes of the single-flow battery and the dual-flow battery are bothporous membranes without ion exchange groups or composite membranes. 3.The zinc-iodine flow battery according to claim 1, wherein the iodinesalt is one or more of CaI2, MgI2, KI, and NaI, and the molarconcentration of the iodized salt in the electrolyte is 2 to 8 mol/L;the zinc salt is one or more of ZnNO3, ZnBr2, ZnSO4, and ZnCl2, and themolar concentration of the zinc salt in the electrolytic solution is 1to 4 mol/L; the molar ratio of iodine and zinc in the electrolyticsolution is preferably between 2:1, wherein zinc salt is preferablyZnBr2, and iodine salt is preferably KI.
 4. The zinc-iodine flow batteryaccording to claim 1, wherein the electrolyte solution contains asupporting electrolyte, and the supporting electrolyte in a single-flowbattery is one or more of KCl, KBr, and NaCl; the electrolyte ofdual-flow battery is one or more of KCl, K₂SO₄, KBr; the concentrationis 1 to 2 mol/L and the supporting electrolyte is preferably KCl.
 5. Thezinc-iodine flow battery according to claim 1, wherein the membrane is aporous membrane without ion-exchange groups, including one or more ofpolyethersulfone (PES), polyethylene (PE), polypropylene (PP),polysulfone (PS), polyetherimide (PEI), and polyvinylidene fluoride(PVDF); the membrane is a porous membrane, and the thickness of asingle-flow battery is 100 to 1000 μm; the thickness of membraneemployed for the dual-flow battery is 150-1000 μm, preferably 500-1000μm; the porous membrane material is preferably PE, PP, pore size is 1-10nm, and porosity is 20%-70%.
 6. The zinc-iodine flow battery accordingto claim 1, wherein the composite membrane is a porous membrane withoution-exchange groups coated with a dense polymer layer on the surface andthe polymer layer material is one or more of polybenzimidazole (PBI),Nafion resins and polytetrafluoroethylene (PTFE), preferably Nafionresin, and the thickness of the coating is 1-10 μm.
 7. The zinc-iodineflow battery according to claim 1, characterized in that during chargingthe I⁻ in the positive electrolyte undergoes an oxidation reaction togenerate one or more of I₃ ⁻ and I₂, preferably I₂; on the negativeelectrode Zn²⁺ undergoes a reduction reaction to form Zn; duringdischarge, the I₃ ⁻ or I₂ undergoes a reduction reaction to generate I⁻on the positive electrode, and the Zn undergoes an oxidation reaction togenerate Zn²⁺.
 8. The zinc-iodine flow battery according to claim 1,wherein the electrode material is one of carbon felt, graphite plate,metal plate, or carbon cloth, preferably carbon felt.